The white blisters on your tomatoes are a highly characteristic and diagnostic symptom of bacterial canker, a serious disease affecting tomato plants. These raised, white lesions on the fruit are often referred to as "bird's-eye spots" due to their distinctive appearance.
Understanding Bacterial Canker
Bacterial canker is a destructive disease caused by the bacterium Clavibacter michiganensis subsp. michiganensis (Cmm). This pathogen can significantly reduce tomato yield and quality. While leaf symptoms might make an observer suspicious of canker, the appearance of these specific white blisters on the fruit is often what confirms the diagnosis.
Key Symptoms of Bacterial Canker on Tomatoes
Bacterial canker can manifest in various ways on different parts of the tomato plant. Recognizing these symptoms early is crucial for effective management.
1. Fruit Symptoms: The White Blisters
- "Bird's-eye Spot" Lesions: These are the most distinctive and diagnostic symptoms on the fruit. They appear as small (1/8 to 1/4 inch in diameter), raised, white to cream-colored blisters.
- Dark Centers: Each blister typically has a slightly sunken, dark brown or tan center, resembling the pupil of a bird's eye.
- White Halo: Often, these spots are surrounded by a translucent white halo, making them stand out against the red or green fruit.
- Surface Level: The lesions are usually superficial, meaning they don't penetrate deeply into the fruit flesh, but they severely impact marketability.
2. Leaf Symptoms
Before or concurrently with fruit symptoms, leaves may show:
- Wilting: Often, wilting starts on one side of a leaf or leaflet and can progress to entire branches.
- Marginal Necrosis: Leaf edges may turn yellow, then brown and crispy, often with a characteristic "scorched" appearance.
- Water-soaked Spots: Small, dark, water-soaked spots can appear on leaves, which may later dry out and fall, leaving holes.
3. Stem Symptoms
- Cankers: Brown streaks or open cankers (cracks) may develop on stems, particularly at nodes.
- Vascular Discoloration: If you cut open an infected stem, the vascular tissue (the inner part that transports water and nutrients) may appear yellow to reddish-brown.
- Stem Splitting: In severe cases, stems can split open, exposing discolored tissue.
How Bacterial Canker Spreads
This pathogen is highly contagious and can spread through several means:
- Infected Seeds and Transplants: The most common way the disease is introduced into a garden or field.
- Contaminated Tools and Equipment: Pruning shears, stakes, and even hands can transfer the bacteria from diseased to healthy plants.
- Water Splash: Rain or overhead irrigation can splash bacterial cells from infected plant parts to healthy ones.
- Wind: Can carry infected plant debris.
Managing and Preventing Bacterial Canker
Early detection and rigorous sanitation are key to controlling bacterial canker. Here are practical steps to protect your tomato plants:
1. Start with Clean Materials
- Certified Disease-Free Seeds: Always purchase seeds from reputable suppliers who certify them as disease-free. Consider hot water treatment for seeds if you're unsure.
- Healthy Transplants: Inspect young plants carefully before planting. Avoid any that show signs of wilting, spots, or discoloration.
2. Implement Good Cultural Practices
- Crop Rotation: Avoid planting tomatoes or related solanaceous crops (like peppers, eggplants, potatoes) in the same spot for at least 2-3 years. This helps reduce bacterial populations in the soil.
- Sanitation:
- Disinfect Tools: Regularly clean and disinfect pruning shears, stakes, and other gardening tools with a 10% bleach solution or rubbing alcohol, especially between plants.
- Wash Hands: Wash your hands thoroughly after handling infected plants.
- Remove Debris: Promptly remove and destroy (do not compost) any infected plant material, including fallen leaves and fruit.
3. Optimize Watering
- Avoid Overhead Irrigation: Use drip irrigation or water at the base of plants to minimize splashing water onto foliage, which can spread bacteria.
- Water in the Morning: This allows leaves to dry quickly, reducing the time bacteria have to infect.
4. Plant Removal
- Immediate Removal: If you identify plants with confirmed bacterial canker (especially with the white fruit blisters), remove them immediately from the garden and dispose of them properly (e.g., burn or double-bag and discard). Do not leave them to infect other plants.
5. Chemical Control (Limited Efficacy)
- Copper-based Sprays: While not curative, preventative applications of copper-based bactericides can offer some protection, especially if applied before significant disease pressure. These are more effective at preventing spread than treating established infections. Always follow label instructions.
Summary of Bacterial Canker Symptoms & Management
| Symptom Location | Appearance | Management Action |
|---|---|---|
| Fruit | Raised, white blisters with dark centers ("bird's-eye spots") | Remove infected fruit and plants immediately |
| Leaves | Wilting, yellowing, marginal browning, water-soaked spots | Avoid overhead watering, sanitize tools |
| Stems | Brown streaks, cankers, splitting, vascular discoloration | Use disease-free seeds/transplants, crop rotation |
By understanding the symptoms and implementing preventative measures, you can significantly reduce the risk and impact of bacterial canker in your tomato garden. For more detailed information on plant disease management, consult resources from your local university extension, such as University of California Agriculture & Natural Resources or Cornell University Cooperative Extension.
